From today, you won't be allowed to pass immigrations in the United States without having an approved ESTA application on file. It's also an extremely good idea to bring a printout to the check in desk, as many airlines will be checking your documentation before allowing you to board a flight. British Airways specifically states you won't be allowed boarding without the clearance.
Electronic System for Travel Authorisation (ESTA) is now mandatory for all travelers from the Visa Waiver countries. However, if you hold a current visa to the U.S., there's no need to fill in an application on the website. You can fill out the form on this website provided by the Department of Homeland Security.
The application process may take up to 15-20 minutes and authorization is normally granted immediately, but may take up to 72 hours. It's advisable to apply for authorization before you even book a flight, since chances are the fare won't be refundable. Once you've got an approved ESTA application, it's valid for another 2 years, or until you get a new passport, whichever is sooner.
So far, it seems the suggested fee for ESTA hasn't been put in place, so as of today you should never have to pay for the clearance.
The ESTA process went live back in 2008, became a recommendation (well, they said it was mandatory then too) in 2009, and is now a real requirement for U.S. travel. In any case, if you're planning a trip to America, get your ESTA now.
If you have any other questions on this requirement, here's how to apply for ESTA clearance.
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